1. Field of the Invention
This invention relates generally to a method and device for measuring the depth of a through-hole formed through one or more workpieces.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
The thickness of sheet material such as composite fiber reinforced plastic (CFRP) can vary from nominal engineering values. Consequently, stack-up thicknesses of multiple sheets of such material and, thus, the depth of fastener holes drilled through a stack of such material varies sufficiently that fastener selection can require that individual hole depth measurements be made. It is, therefore, advantageous to be able to quickly and accurately measure the depths of holes that have been formed through sheet material stacks so that respective fasteners of appropriate grip length can be selected for use in the holes. It's also advantageous to be able to determine such hole depths and corresponding fastener grip lengths in advance of fastener selection and installation, and without having to execute a separate measurement step following the drilling of each hole. Accurate individual hole depth measurements can also allow a fastener bill of materials (BOM) to be prepared based on actual rather than nominal hole depths, which would allow fasteners to be delivered and kilted in advance and delivered in proper quantities to an assembly station. However, known through-hole depth measurement methods and devices are unable to sense changes in drill force in a drill that comprises a pressure foot system—such as one driven by an air cylinder—that's configured to exert a constant pressure against the facing surface of a workpiece.